The reproduction of graphic images by printing methods such as letterpress or offset lithography, for example, is effectively a binary process, that is one decides simply whether or not to put down opaque ink on a particular small area of the printing stock. In general, it is not practicable to reproduce tone variations by controlling the amount of ink applied at any point.
Traditionally the technique of optical screening has been used to reproduce tone variations. A screen consisting of a mesh of strips of controlled opacity is placed between the illuminated original image recorded on film and some unexposed photo-sensitive material. Diffraction at the screen, causes the image to break up into small regions known in the art as dots, whose area corresponds to the local optical density of the original. The dot image is recorded on the sensitive surface and forms, after development, what is commonly known as a half-tone image. Those skilled in the art will know of the many different types of optical screen that are available and the alternative methods for locating the screen with respect to the original image, as well as the need for high contrast recording of the diffracted image.
Recently electronic apparatus has become available for processing graphic images for printing and related reproduction techniques, and those familiar with these systems will be aware that electronic equivalents of the optical screening process can be devised so that screening may be accomplished by electronic modification of the electrical signal that represents the image within such apparatus.
Most of the electronic systems referred to above employ a cathode ray tube to form the final image on the photosensitive material, which is subsequently used to make the actual printing surface. For example an electronic screening method has been proposed which involves the generation of half-tone dots in the final image by means of a micro-scanning pattern on the face of the cathode ray tube. Electronic devices are used to provide special waveforms for the control of the cathode ray tube light spot, so that the latter generates a series of dot-images analogous to the rows of dots that would be formed by an optical screen.